The present invention relates to a catalyst composition and a process for the di-, tri- and/or tetramerization of ethylene.
Existing processes for the production of linear alpha olefins (LAOs), including comonomer-grade 1-hexene and 1-octene, rely on the oligomerization of ethylene. These processes have in common that they lead to a product distribution of ethylene-oligomers of chain length 4, 6, 8 and so on. This is due to a chemical mechanism which is widely governed by competing chain growth- and displacement reaction steps, leading to a Schulz-Flory- or Poisson-product distribution.
From the marketing point of view, this product distribution poses a formidable challenge for the full-range alpha olefins producer. The reason is that each market segment served exhibits a very different behavior in terms of market size and growth, geography, fragmentation etc. It is, therefore, very difficult for the producer to adapt to the market requirements since part of the product spectrum might be in high demand in a given economic context, while at the same time other product cuts might not be marketable at all or only in a marginal niche. Currently, the highest-value LAO product is comonomer-grade 1-hexene for the polymer industry, while 1-octene demand is also growing at a considerable rate.
Thus, the on-purpose production of the most economically viable LAOS, i.e., comonomer-grade 1-hexene and 1-octene, appears highly desirable. To meet the requirements regarding high C6- and/or C8-selectivities, new processes have been developed. The only known selective C6-commercial process has been commissioned by Chevron Phillips, see for a comprehensive review e.g. J. T. Dixon, M. J. Green, F. M. Hess, D. H. Morgan, “Advances in selective ethylene trimerisation—a critical overview”, Journal of Organometallic Chemistry 689 (2004) 3641-3668.
Furthermore, patent applications have been filed by Sasol (WO 93/053891 A1), disclosing chromium-based selective ethylene-trimerization catalyst systems, typically of the type CrCl3 (bis-(2-diphenylphosphino-ethyl)amine)/MAO (methylaluminoxane). Also disclosed were variations of the ligand structure (e.g. bis(2-diethylphosphino-ethyl)amine, pentamethyldiethylenetriamine etc.). However, all these complexes generate considerable amounts of unwanted side products such as LAOs other than 1-hexene and polyethylene.
A large body of scientific publications and patent literature describes the use of chromium-based metal-organic complexes with ligands featuring the basic PNP-structure (for example bis(diphenylphosphino)amine-ligands) (D. S. McGuinness, P. Wasserscheid, W. Keim, C. Hu, U. Englert, J. T. Dixon, C. Grove, “Novel Cr-PNP complexes as catalysts for the trimerization of ethylene”, Chem. Commun., 2003, 334-335; K. Blann, A. Bollmann, J. T. Dixon, F. M. Hess, E. Killian, H. Maumela, D. H. Morgan, A. Neveling, S. Otto, M. J. Overett, “Highly selective chromium-based ethylene trimerisation catalysts with bulky diphosphinoamine ligands”, Chem. Comm., 2005, 620-621; M. J. Overett, K. Blann, A. Bollmann, J. T. Dixon, F. Hess, E. Killian, H. Maumela, D. H. Morgan, A. Neveling, S. Otto, “Ethylene trimerisation and tetramerisation catalysts with polar-substituted diphosphinoamine ligands”, Chem. Commun., 2005, 622-624; A. Jabri, P. Crewdson, S. Gambarotta, I. Korobkov, R. Duchateau, “Isolation of a Cationic Chromium(II) Species in a Catalytic System for Ethylene Tri- and Tetramerization”, Organometallics 2006, 25, 715-718; T. Agapie, S. J. Schafer, J. A. Labinger, J. E. Bercaw, “Mechanistic Studies of the Ethylene Trimerization Reaction with Chromium-Diphosphine Catalysts: Experimental Evidence for a Mechanism Involving Metallacyclic Intermediates”, J. Am. Chem. Soc. 2004, 126, 1304-1305; S. J. Schafer, M. D. Day, L. M. Henling, J. A. Labinger, J. E. Bercaw, “Ethylene Trimerization Catalysts Based on Chromium Complexes with a Nitrogen-Bridged Diphosphine Ligand Having ortho-Methoxyaryl or ortho-Thiomethoxy Substituents: Well-Defined Catalyst Precursors and Investigations of the Mechanism”, Organometallics 2006, 25, 2743-2749; S. J. Schofer, M. D. Day, L. M. Henling, J. A. Labinger, J. E. Bercaw, “A Chromium-Diphosphine System for Catalytic Ethylene Trimerization: Synthetic and Structural Studies of Chromium Complexes with a Nitrogen-Bridged Diphosphine Ligand with ortho-Methoxyaryl Substituents”, Organometallics 2006, 25, 2733-2742; P. R. Elowe, C. McCann, P. G. Pringle, S. K. Spitzmesser, J. E. Bercaw, “Nitrogen-Linked Diphosphine Ligands with Ethers Attached to Nitrogen for Chromium-Catalyzed Ethylene Tri- and Tetramerization”, Organometallics 2006, 25, 5255-5260; WO 2004/056578, WO 2004/056479, EP 02 794 480.0, EP 02 794 479.2; or the SNS-structure (D. S. McGuinness, D. B. Brown, R. P. Tooze, F. M. Hess, J. T. Dixon, A. M. Z. Slavin, “Ethylene Trimerization with Cr-PNP and Cr-SNS Complexes: Effect of Ligand Structure, Metal Oxidation State, and Role of Activator on Catalysis”, Organometallics 2006, 25, 3605-3610; A. Jabri, C. Temple, P. Crewdson, S. Gambarotta, I. Korobkov, R. Duchateau, “Role of the Metal Oxidation State in the SNS-Cr Catalyst for Ethylene Trimerization: Isolation of Di- and Trivalent Cationic Intermediates”, J. Am. Chem. Soc. 2006, 128, 9238-9247; C. Temple, A. Jabri, P. Crewdson, S. Gambarotta, I. Korobkov, R. Duchateau, “The Question of the Cr-Oxidation State in the {Cr(SNS)} Catalyst for Selective Ethylene Trimerization: An Unanticipated Re-Oxidation Pathway”, Angew. Chem. Int. Ed. 2006, 45, 7050-7053); for both, trimerization and tetramerization of ethylene. Excess amounts of MAO are most commonly used as activator/co-catalyst.
While the majority of the published studies rely on Cr—PNP complexes, some deal with other ligands, e.g. of the general formula (R1)(R2)P—X—P(R3)(R4), where X is a bivalent organic bridging group, see WO 2005/039758 A1, or deal with entirely different complexes, such as titanocenes (H. Hagen, W. P. Kretschmer, F. R. van Buren, B. Hessen, D. A. van Oeffelen, “Selective ethylene trimerization: A study into the mechanism and the reduction of PE formation”, Journal of Molecular Catalysis A: Chemical 248 (2006) 237-247). In either case, the major concern is always selectivity and minimization of polyethylene formation.
The ethylene trimerization and tetramerization catalysts and processes disclosed so far in scientific and patent literature generally have one or more of the following disadvantages:                Low selectivities to the desired products 1-hexene and/or 1-octene (undesired byproducts from side reaction channels).        Limited purities of the products, i.e. the selectivities within the specific C6- or C8-cut (isomerization, branched olefin formation etc.)        Wax formation, i.e. formation of heavy, long-chain, high carbon-number products.        Polymer formation (polyethylene, branched and/or cross-linked PE); this leads to considerable product yield loss and fouling of equipment.        Poor turnover rates/catalyst activity, resulting in high cost per kg product.        High catalyst—or ligand cost.        Difficult ligand synthesis, resulting in poor availability and high catalyst cost.        Susceptibility of catalyst performance, in terms of both activity and selectivity, to trace impurities (catalyst losses/-poisoning).        Difficult handling of catalyst components in a technical environment (catalyst complex synthesis, pre-mixing, inertization, catalyst- or ligand recovery).        Harsh reaction conditions, i.e. high temperatures and pressures, resulting in high investment-, maintenance-, and energy cost.        High co-catalyst/activator cost and/or consumption.        Susceptibility to varying co-catalyst qualities; often the case when larger amounts of relatively ill-defined compounds must be used as activators (e.g. certain MAO-varieties).        